Portable entertainment devices have become common and include portable devices that generate audio output only, which are referred to herein as portable audio devices, as well as portable devices that generate both audio and video outputs, which are referred to herein as portable audiovisual devices. Examples of portable audio devices include digital music players, compact disc players, and cassette tape players. Examples of portable audiovisual devices include video game consoles, portable DVD players, and portable digital video players.
The voltages of the audio output signals provided by different portable entertainment devices is not standardized, which can cause volume discrepancies during playback. For example, a portable audio device may provide an output signal at a voltage of 300 mV, while a portable audiovisual device may provide its audio output at a voltage of approximately 1 V.
In response to the proliferation of portable entertainment devices, it has become common to include means for interfacing with portable entertainment devices in the audiovisual systems of vehicles. In a vehicle, volume discrepancies can cause driver distraction. For example, if an audio signal having a voltage of 1 V is provided to an automobile audiovisual system that is calibrated to receive input signals at 300 mV, the automobile audiovisual system will produce an abnormally loud output volume. If this occurs while a driver is operating the vehicle, the driver could become distracted by the unexpected change in volume. Conversely, if an audio signal having a voltage of 300 mV is provided to an audiovisual system that is calibrated to receive an audio input signal at 1 V, the resulting output volume will be abnormally soft, and the driver will likely increase the volume setting. Subsequently, when a different input source is selected, the automobile audio system may produce an abnormally loud output.
One known method for accommodating audio input signals of differing voltages is to provide a manual switch, by which a user may manually change the input sensitivity of the audiovisual system. While effective, an affirmative action by the user is required, and the user may not remember to adjust the input sensitivity prior to beginning operation of the portable audiovisual device. Another method involves detecting the voltage of the audio signal provided by the portable audiovisual device and adjusting the input sensitivity of the audiovisual system accordingly. However, this solution requires complicated circuitry that increasing the total cost of the vehicle's audiovisual system.